1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
2. Related Background Art
An electrophotographic image forming apparatus, such as a copying machine, includes a photoreceptor for printing a visible image, formed by toner, on paper serving as a recording medium (object to be printed on), and a developing unit for supplying toner to the photoreceptor using a development roller (magnet roller). The developing unit may include a two-component developing unit using a mixture of a non-magnetic toner and a magnetic carrier as a developer. In recent years, the demand for high-quality image is increased, particularly in the field of full-color electrophotographic image forming apparatuses. As a result, the reduction in particle size of magnetic carriers is being advanced. A problem with the reduction in particle size of carriers lies in that the margin of white background contrast potential (potential difference between the white background potential of the photoreceptor and the development bias of the development roller) is reduced. If the white background potential (potential difference) is too large, the carrier particles tend to adhere to the photoreceptor, resulting in that scratches and abrasions may appear on the photoreceptor. On the contrary, if the white background potential is too small, the entire paper may be darkened (fogged). Because of this, generally the white background contrast potential (potential difference) is set to be at a value satisfying both the carrier adherence property and the fogging property. However, when the size of carrier particle is reduced, the margin of the white background contrast potential (potential difference) is also reduced. Because of this, it is difficult to use carriers having a smaller particle size.
In order to solve this problem, conventional electrophotographic image forming apparatuses have been provided with a sensor for detecting the potential of the photoreceptor to achieve a high-accuracy white background potential, thereby adjusting the white background contrast potential (potential difference) so as to be within the margin. This enables the use of carriers with smaller particle size. However, the existence of the photoreceptor potential sensor leads to an increase in both the size and the cost of the apparatuses. This goes against the recent tendency of reduced size and lower cost. In particular, in the field of four-drum tandem color electrophotographic image forming apparatuses, such a problem is critical since there are four photoreceptors in these apparatuses.
The present invention is proposed in consideration of the above-described problem, and the object of the present invention is to provide a high image quality electrophotographic image forming apparatus, which can use carriers with smaller particle sizes without including any sensor system, which inevitably increases the size of the apparatus, and without increasing the costs.
According to embodiments of the present invention, there is provided an electrophotographic image forming apparatus comprising: a photoreceptor which is rotatable, and a surface of which can be uniformly charged at a voltage level, the surface attracting a toner for printing with an electrostatic force, the toner being transferred to an object to be printed on, such as paper, to perform an printing operation, and said photoreceptor rotating predetermined times during the printing operation; a charging unit for charging the surface of the photoreceptor to be either at a first white background potential or at a second white background potential; an exposing unit for forming an electrostatic print pattern on the surface of said photoreceptor set to be at the second white background potential, to form a latent image; a developing unit having a rotatable development roller, which rotates with a developer adhering to the surface thereof, the developer including a carrier and the toner attracting each other by an electrostatic force, the toner adhering to the latent image on the surface of the photoreceptor by the electrostatic force, a development bias being applied to the development roller, a potential difference between the development bias and the first white background potential being a first contrast potential, a potential difference between the development bias and the second white background potential being a second white background contrast potential, and an absolute value of the second white background contrast potential being greater than an absolute value of the first white background potential; and a control unit for controlling the charging unit so that a part of a total surface area corresponding to all the turns of the photoreceptor during a printing operation is charged to be at the second white background potential, the part corresponding to a predetermined number of turns relevant to the printing operation, and that the rest of the total surface area corresponding to the rest of turns is charged to be at the first white background potential.
According to embodiments of the present invention, there is provided a development method using an electrophotographic image forming apparatus, comprising: rotating a photoreceptor predetermined times, charging a first part of the total surface area corresponding to all the turns of the photoreceptor to be at a first white background potential by the use of a charging unit, the first part being irrelevant to the printing operation; charging a second part of the total area of the photoreceptor relevant to the printing operation to be at a second white background potential, the value of the second white background potential being different from that of the first white background potential; forming an electrostatic pattern of a latent image on the second part of the total surface area of the photoreceptor charged to be at the second white background potential by the use of an exposing unit, supplying a developer composed of a toner and a carrier attracting each other by an electrostatic force to the latent image on the second part of the total surface area of the photoreceptor by the use of a development roller charged to be at a development bias potential; and allowing the toner to adhere to the latent image on the second part of the total surface of the photoreceptor by the electrostatic force, wherein with respect of the absolute value of the potential difference between the photoreceptor and the development roller, the absolute value of a second contrast potential, which is the potential difference between the second white background potential of the second part relevant to the printing operation and the development bias potential of the development roller is larger than the absolute value of a first contrast potential, which is the potential difference between the first white background potential of the first part irrelevant to the printing operation and the development bias potential.